Newsletter

Know the Snow: Spring in the backcountry

A few days ago I went for a short backcountry tour with my wife. After a brief climb from the Eaglecrest Ski Area parking lot, we reached the bottom of the Wedding Bowl and started to ponder where to ski. We were too late to ski east-facing slopes. These aspects have seen too much sun already and would be too wet to ski safely. South- and west-facing slopes were still frozen. Slopes facing north had dry winter snow with crust on top. Without too much time to time to kill, we could not wait for the southerly aspect to thaw and soften. We decided to gain more elevation and ski a southeast facing slope. We kept climbing to where the snow surface just started to thaw and skied down. Still, by the time we reached lower elevations, we were skiing softer snow than we wanted to.

Like in many facets of life, managing spring snow is all about timing and place. The difference between great corn, terrible crust, sinking into deep slush or wet avalanches can be the extra half an hour you spent in bed in the morning.

From all types of spring snow, corn snow is by far my favorite. It feels smooth, creamy and makes a lovely sound under my skis. Corn snow develops when the upper parts of the snow go through several melt -freeze cycles. When the snow becomes wet, it goes through a cannibalistic process where large ice grains grow larger at the expense of smaller grains. After a few melt-and-freeze cycles the ice grain reaches the size of a corn seed. When the time is right, the top inch or two of the snow surface is grainy, loose and ready to surrender under the forces inflected on it by skis or snowboard. Below the surface, the snowpack is still frozen and supports the snow rider on the snow surface. Over time under the sun, the supporting crust melts and loses its ability to prevent the rider from sinking into the wet snow.

When snow gets very wet it loses its strength and can avalanche. Wet snow avalanches typically occur when the dry winter snow becomes wet for the first time. This snow usually soaks water like a sponge, becoming saturated and weak. After the wet snow develops into large ice crystals, it also develops large pores. These pores quickly channel the water flow toward the ground without dissolving many of the bonds between the ice crystals. This is the reason that wet avalanche cycles tend to start on warm, sunny slopes and slowly migrate toward colder slopes throughout the spring. Wet avalanches usually start to appear on sunny low elevation slopes. As the season progresses and temperatures rise, these avalanche cycles migrate toward higher elevations and less sunny aspects. Slopes that survive the wet avalanche wave usually develop channels to drain snow melt water and become stable within a couple of days.

Clearly, not every dry snowpack is conducive to avalanching when it gets wet. Otherwise, there would be no snow left on the slope after the first thaw.

The obvious question is: what kind of snowpack is capable of producing wet slab avalanches?

Wet slab avalanches typically occur when percolating water through the snowpack reaches an impenetrable snowpack layer and starts to flow along its plane, dissolving more and more bonds along the layer’s plane. In time, large areas along the flow plane lose their holds and a fracture develops. Over the years, we got used to thinking about ice layers as the likely cause for wet slab avalanche failure. However, when Erich Peitzsch and Karl Birkeland took a hard look on wet slab avalanches they found that these avalanches tend to fail on interfaces between layers with small grains above larger grains. Water tends to flow along these interfaces in the same way it flows along the interior of a tent’s rain fly.

Skilled avalanche folks usually know what avalanches the spring will bring on different slopes well before the big thaw starts. They track where and when wet slab avalanches occur. They pay close attention to the water flow inside the snowpack on aspects and elevations that didn’t avalanche yet and see how it flows through the snowpack.

• Ron Simenhois is an avalanche forecaster who lives in North Douglas. You may contact him at simenhois.ron@gmail.com.